The invention relates to a method as recited in the preamble of claim 1. U.S. Pat. No. 4,559,625 to Berlekamp et al and U.S. Pat. No. 5,299,208 to Blaum et al disclose the decoding of interleaved and error protected information, where an error pattern found in a first word may give a clue to locate errors in another word of the same group of words. Errors pointed at are relatively closer or more contiguous than other symbols of the word that would generate the clue. The references use a standardized format and a fault model with multisymbol error bursts across various words. Occurrence of an error in a particular word may imply a strong probability for an error to occur in a symbol pointed at in a next word or words. The procedure will often raise the number of errors that may be corrected before the mechanism fails.
The present inventor has recognized a problem with this method: a clue will materialize relatively late in the process when the clue originating information will have been demodulated as well as been fully corrected. This will complicate the use of higher level measures such as a retry read of the data during a later disc revolution. Also, the present inventor has recognized that part or all of the clues could be acquired against a lower investment in terms of the amount of redundancy.